Layered double hydroxides (LDH) are a class of layered compounds having anion exchangeability represented by the formula:[M2+1-xM3+x(OH)2]x+[An−x/nyH2O]Their crystal structure consists of a plurality of octahedral hydroxide layers (basal layers) in which a portion of bivalent metal ions is replaced with trivalent metal ions in the hydroxide, and a plurality of intercalation layers consisting of anions and water present between adjacent basal layers. The characteristic feature of LDH's resides in a wide variety of the species and ratios of metal ions of the basal layer and the combination of the basal layer and the intercalation layer. A large number of LDH's have been synthesized to date and their entrapment of various inorganic and organic anions has been studied.
The basal layers of LDH in general have a relatively high electrostatic charge density and, therefore, the electrostatic attraction force between the basal and intercalation layers are strong enough so that delamination could hardly occur like many clay minerals. Accordingly, only a few reports address an LDH capable of delamination in water. JP 2004-189671A discloses an LDH intercalated with an aromatic amino carboxylic acid such as p-aminobenzoic acid that delaminates in water or a lower alcohol such as ethanol to form a dispersion of delaminated LDH. It is explained that the delamination occurs as a result of enlarged spacing distance between the basal layers by the intercalated aminocarboxylic acid molecule having a larger molecular size than CO32− ion. However, the delamination of this LDH is complete in a good solvent of the aromatic aminocarboxylic acid such as p-aminobenzoic acid but not complete in water in which the aromatic aminocarboxylic acid is less soluble. A need exists for a new type of LDH that delaminates in water almost completely.